Identification of common and distinct residues involved in the interaction of alphai2 and alphas with adenylyl cyclase.

The G protein alpha subunits, alphas and alphai2, have stimulatory and inhibitory effects, respectively, on a common effector protein, adenylyl cyclase. These effects require a GTP-dependent conformational change that involves three alpha subunit regions (Switches I-III). alphas residues in three adjacent loops, including Switch II, specify activation of adenylyl cyclase. The adenylyl cyclase-specifying region of alphai2 is located within a 78-residue segment that includes two of these loops but none of the conformational switch regions. We have used an alanine-scanning mutagenesis approach within Switches I-III and the 78-residue segment of alphai2 to identify residues required for inhibition of adenylyl cyclase. We found a cluster of conserved residues in Switch II in which substitutions cause major losses in the abilities of both alphai2 and alphas to modulate adenylyl cyclase activity but do not affect alpha subunit expression or the GTP-induced conformational change. We also found two regions within the 78-residue segment of alphai2 in which substitutions reduce the ability of alphai2 to inhibit adenylyl cyclase, one of which corresponds to an effector-activating region of alphas. Thus, both alphai2 and alphas interact with adenylyl cyclase using: 1) conserved Switch II residues that communicate the conformational state of the alpha subunit and 2) divergent residues that specify particular effectors and the nature of their modulation.